ADHI-4899: Initial implementation of mig_is_server

This adds new application mig_is_server which implements a small part of is_server interface used by HLT providers. Internally it sends objects to a gatherer which merges them and pushes to a downstream is_server. Objects that cannot be sent to gatherer are re-published to a downstream is_server. This new application can become a replacement for gatherer part of ipc_proxy.